1. An ever increasing amount of evidence shows that homosexual desire (and heterosexual desire for that mater) is motivated primarily by biological factors such as differences in the way the brain is physically structured

2. Activists, seizing on this evidence, have argued that because it is almost certainly the case that the majority of homosexually oriented people are born, not recruited, it should follow that the old "moral" prejudices against homosexuality should fall away.

3. Saletan argues, however, that the prejudice will not go away, but merely change its shape. Homophobics will no longer argue that homosexuality is a moral problem, but instead come to look at it as a biological defect which should be treated medically with increasingly sophisticated and early-life interventions including genetic screening, manipulations of the intrauterine hormonal environment during pregnancy and similar techniques.

I dearly hope Saletan's argument is wrong, but I kinda think he's on to something. Homophobia is deeply ingrained into the culture we live in. Some people saddled with homophobic prejudices will surely leap at the opportunity to actually manipulate the structure of the developing brain so as to prevent the occurrence of the "gay disease". It isn't possible to do that just yet, but the writing is now on the wall, and it appears only a matter of time and will (and the willing suspension of ethical precepts that would prevent messing with developing fetuses) before such treatments are medically possible.

In order to talk about the shape such treatments might take, we need to unpack the biological processes and structures that recent studies are suggesting shape sexual desire. In the last week or so, two interesting studies have addressed this topic.

The first study, titled "PET and MRI show differences in cerebral asymmetry and functional connectivity between homo- and heterosexual subjects", by Ivanka Savic and Per Lindstrom and published in the Proceedings of the National Academy of Sciences used brain scans to examine differences in brain structures between groups of heterosexual and homosexual subjects and found a most interesting pattern which can be boiled down to this: Aspects of gay male subjects' brains were more similar to those of straight females than to straight males. Vice versa, aspects of gay females' brains were more similar to straight males than to straight females. The amygdala, a sub-cortical structure (meaning one of the evolutionarily older parts of the brain) known to be a center of emotional responding, followed this pattern, as did other parts of the brain playing important roles in judgment and language processing such as cortical hemispheric asymetries (whether the two sides of the outer brain are the same size or not). Regarding cortical asymetries, the normal case is that women tend to have similarly sized brain halves, while in men, the right side of the brain (important for non-verbal spatial processing) is in most cases larger than the left side of the brain which tends to be more important for language skills.

Though the brain does grow throughout childhood and adolescence, the relative shapes and sizes of these various brain components do not change much, according to researchers who are in a position to know. The conclusion is that when these areas are differently shaped, that shaping largely occurred while the brain was being formed during pregnancy.

Though many forces contribute to shaping the brain during its development, the role of genetics and maternal hormones is large in this regard. Genes provide the basic blueprints for building the brain, while hormones including the sexual hormones testosterone and estrogen play a switching role during the developmental process, turning on and off at the appropriate times different sequences of genetic instruction. Exactly what pattern of genetics and/or hormones is responsible for structuring someone's brain into a heterosexual mold or a homosexual mold is not know yet and remains a topic of active study.

Another new study recently released speaks directly to this issue of which is more important, genes or hormones in shaping homosexuality. The answer according to the study authors is that, at least with regard to homosexual men, the evidence points to a genetic role.

that homosexuality has been a low frequency but constant feature of all studied human societies

that male homosexuality tends to run in the family on the maternal line of homosexual men.

that the mothers of homosexual men tend to end up having more children on average than similar mothers with all heterosexual children, and

that the close female relatives of mothers of homosexual men also tend to have more children on average than the female relatives of fathers of homosexual men.

The explanation, descriptively named sexually antagonistic selection, suggests that there is a gene or cluster of genes that is inherited through the maternal line. This same gene or cluster of genes is responsible for enhancing female fertility and for producing some minority of male offspring who become homosexually oriented. Though many homosexual men might not choose to reproduce (or find themselves in a life circumstance where this failure to reproduce happens as a matter of course) and would not pass this gene on to future generations, the gene persists because of the enhanced fertility advantage it confers to female who have it. The extra children these women end up producing, most of whom end up heterosexual and motivated to reproduce, more than offset the loss of homosexually orientated children's relative lack of reproductive motivation.

For more on this study, check out these good analyses in Slate, and in ThinkGene.

This explanation does not fit what is known about female homosexually oriented people, and thus a genetic explanation of lesbianism is not supported by the data. In the absence of a genetic explanation, the hormonal explanation would seem more likely at present as a primary cause of sexual orientation.

If there is a "gay gene", at least with regard to males, you can be sure that it will be identified within a few years from now. If homosexuality is influenced by maternal hormones, you can similarly be sure that we will know what those homones are and in what doses they do their thing in short order. And then what? A quote from the Sexual Reorientation article makes the point explicitly clear:

Would hormonal intervention work in humans? Should we try it? Some thinkers are intrigued. Last year, the Rev. Albert Mohler Jr., president of the Southern Baptist Theological Seminary, wrote: "If a biological basis is found, and if a prenatal test is then developed, and if a successful treatment to reverse the sexual orientation to heterosexual is ever developed, we would support its use." Mohler told the Associated Press that morally, this would be no different from curing fetal blindness or any other "medical problem". The Rev. Joseph Fessio, editor of the press that publishes the pope's work, agreed: "Same-sex activity is considered disordered. If there are ways of detecting diseases or disorders of children in the womb … that respected the dignity of the child and mother, it would be a wonderful advancement of science."

It is already possible to do pre-implantation genetic testing (commonly known as PGD) on fertilized embryos scheduled for in-vitro fertilization. Some reproductive medicine specialists regard PGD, only half jokingly, as the future of all human reproduction. In PGD, a cell is plucked off of the not-yet-implanted developing embryo, and the chromosomes are extracted from that cell and fed through a set of tests that scans them for markers indicating the presence of genes associated with undesirable medical conditions. If a particular embryo is found to be defective, it gets discarded rather than implanted or frozen for later implantation. PGD is currently an expensive procedure but one widely available to infertile couples who do not want to risk the possibility of an disease-prone baby. What, other than the ethics of the reproductive doctors themselves and the cost involved will prevent the use of PGD and similar techniques by homophobic parents to identify and discard male embryos who might ultimately turn out to be gay before they can be implanted?

Lesbians should not feel any safer than gay men. Control over maternal hormones during pregnancy is also a highly developed aspect of reproductive medicine. Normal female reproductive hormones cycles are routinely suppressed and artificially regulated during the course of in-vitro fertilization. Almost complete control is taken over the hormonal systems that induce women's eggs to grow and be released into the fallopian tubes. It requires only a short stretch of the imagination to visualize a day not very far in the future when the hormonal patterns that lead towards homosexuality are well known and can be suppressed or "corrected" so as to produce a more "well adjusted" child.

I don't mean to be alarmist and hope that I don't come across that way. The enormous expense of PGD and assisted reproduction alone will ensure that only the most motivated and wealthy parents will ever be able (in the foreseeable future) to avail themselves of technologies capable of suppressing nature's plan for each child. Attractive though the idea of genetic purity is to some people, in practice it will be very expensive to achieve a Gattaca style world. There is also the encouraging fact that the (American) population appears to be becoming progressively moretolerant of homosexuality as a normal part of life. For every American that finds a way to cling to their prejudice, it seems that several others are laying theirs down.

It may seem odd that I'm spending so much time on what seems to be a cultural or neuroscience issue when this is a mental health website. To my mind, however, negative cultural attitudes towards homosexuality are very much a mental health issue inasmuch as it ends up being internalized as shame and self-depreciating beliefs that help to promote depression and unhappiness and in some prominent cases, the compulsive desire to persecute other homosexuals as a method for denying one's own homosexuality. On this latter subject, re-watch the excellent 1999 movie American Beauty, or read between the lines in accounts of the lives of Roy Cohn or Senator Larry Craig. The social emotion shame and the various ways that people internalize negative societal attitudes such that they become embarrassed and ashamed, and begin to persecute and harshly judge themselves is a hugely important contributer to many people's depression, and one that is seriously under-estimated in terms of how important it is. We can only hope, in this regard, that as time passes the culture at large will continue to become more accepting and less uptight regarding homosexuality; that Saletan is right for some but misjudged the larger popular opinion. Nobody deserves to feel like they are damaged goods simply because of who they, as one consenting adult engaged with another consenting adult, choose to sleep with.